My check engine light came on as I drove off. Yes I hit it hard, it's a r/t. on 2011 Dodge Charger

After the light came on, it slowed down like it was in neutral.I made it home, and turned it off, and turned it back on to see if it would start. It did, but my light is still on.Can you tell me what's wrong.I took it to auto zone, the told me it was my gear rato. If you think that's correct, can you explain to me the is it serious and exactly what it is. Thank you